DOC: Improve DataFrame.select_dtypes examples

[tuliocasagrande]

Greetings,

This is a minor change in the DataFrame.select_dtypes doc.

• Add formatting (such as in np.number), to illustrate better it is a piece of code.
• Add decimal place to the third column of the example to proper address it is a float column (maybe in the past float numbers didn't show decimal places all the time, but now they do).

Thanks for reviewing this!
Let me know if I can help further more.

[codecov]

Codecov Report

Merging #19188 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #19188      +/-   ##
==========================================
+ Coverage   91.52%   91.54%   +0.02%     
==========================================
  Files         147      147              
  Lines       48775    48775              
==========================================
+ Hits        44639    44651      +12     
+ Misses       4136     4124      -12

Flag	Coverage Δ
#multiple	89.91% <ø> (+0.02%)	⬆️
#single	41.62% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/frame.py	97.62% <ø> (ø)	⬆️
pandas/plotting/_converter.py	66.95% <0%> (+1.73%)	⬆️

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[jreback]

thanks!